1. Field of the Invention
The present invention relates to a method of manufacturing a liquid crystal display device by a one-drop-fill process, and more particularly to a method of manufacturing liquid crystal displays whereby the plurality of liquid crystal display devices are simultaneously formed by use of large substrates.
2. Description of the Prior Art
Liquid crystal display devices have been widely used for various kinds of electronic equipment since liquid crystal displays are thin and light and have merits of being driven with low voltage and consuming less electric power.
Common liquid crystal displays used for televisions and personal computers are constituted such that liquid crystals are enclosed between two transparent substrates disposed opposite each other. A pixel electrode and a thin film transistor (TFT) are formed on each pixel in one substrate, and color filters facing the pixel electrodes and a common electrode shared by each pixel are formed on the other substrate. Furthermore, polarizing plates are adhered to the opposite sides of the facing sides of each transparent substrate.
In the liquid crystal display device thus constituted, application of voltage between the pixel electrodes and the common electrode changes the directions of liquid crystal molecules located between the pixel electrodes and the common electrode. As a result, light transmittance changes. By controlling the light transmittance for each pixel, it is possible to display a desired image on the liquid crystal display device. Hereinafter, the substrate on which the pixel electrodes and the TFTs are formed is referred to as a TFT substrate, and the substrate on which the color filters and the common electrode are formed is referred to as a CF substrate.
There are two methods of enclosing liquid crystals between the TFT and the CF substrates, a vacuum infusion process and a one-drop-fill process. The advantage of the one-drop-fill process is that the operation time thereof is shorter than that of the vacuum infusion process.
FIGS. 1 to 3 are diagrams illustrating a method of manufacturing a liquid crystal display device by the one-drop-fill process. Note that columnar spacers which maintain uniform cell gaps are formed on the CF substrate in the examples below.
First, as shown in FIG. 1, a dispenser applies ultraviolet curing sealant 81 to make a quadrilateral frame shape as to enclose a display area of the TFT substrate 80. Thereafter, liquid crystals 82 are dropped in the area enclosed by the sealant 81. In this case, as shown in FIG. 1, liquid crystals 82 are dropped in a dispersed way on a plurality of spots in the area enclosed by the sealant 81. Note that the sealant 81 can be applied to both/either the TFT substrate or the CF substrate. In addition, the liquid crystals 82 are dropped either on the TFT substrate or on the CF substrate.
Second, as shown in FIG. 2A, a TFT substrate 80 and a CF substrate 85 are respectively attached to level blocks 91 and 92 in a chamber of a assembling device 90. In this example, since the liquid crystals 82 have already been dropped on the TFT substrate 80, the TFT substrate 80 is attached to the lower level block 91, and the CF substrate 85 is attached to the upper level block 92.
Third, as shown in FIG. 2B, the inside of the chamber of the assembling device 90 is evacuated. Thereafter, the TFT substrate 80 and the CF substrate 85 are temporarily glued with the sealant 81 by moving down the level block 92 (shown in FIG. 2C) after positioning the TFT substrate 80 and the CF substrate 85 by use of a camera (not shown). In this step, the liquid crystals 82 are spread in a space enclosed by the TFT substrate 80, the CF substrate 85 and the sealant 81. In addition, the resinous columnar spacers provided on the CF substrate 85 maintain a constant interval between the TFT substrate 80 and the CF substrate 85. Hereinafter, the structure constituted by bonding the TFT substrate 80 and the CF substrate 85 is referred to as a panel 87.
Fourth, as shown in FIG. 2D, the inside of the chamber of the assembling device 90 is recovered to atmospheric pressure to remove the panel 87. Thereafter, as shown in FIG. 2E, the panel 87 is irradiated with light from an ultraviolet (UV) lamp 88 to cure the sealant 81. Thus, the liquid crystals 82 are enclosed between the TFT substrate 80 and the CF substrate 85.
Incidentally, when removing the panel 87 from the assembling device 90, the sealant 81 is not cured yet. Accordingly, when recovering the inside of the chamber of the assembling device 90 to atmospheric pressure, the atmospheric pressure is quickly applied on the sealant 81, which is not cured. Consequently, when adhesion is insufficient between the sealant 81 and the TFT substrate 80 or the CF substrate 85, air enters inside the panel 87 from where the adhesion is insufficient and causes faulty display.
In addition, as shown in FIG. 3, when the inside of the chamber is recovered to the atmospheric pressure, downward force caused by the atmospheric pressure is applied to the area enclosed by the sealant 81. However, the downward force is not applied to the outside of the area enclosed by the sealant 81. Thus, although the cell gaps determined by the height of the spacers 86 are maintained in the area where the spacers 86 are provided, desired cell gaps cannot occasionally be maintained due to distortion of the substrate in the vicinity of the sealant 81 where the spacers 86 are not provided. This degrades the display quality in the vicinity of the sealant 81.
In order to prevent such a drawback to occur, double application of a sealant has been proposed in Japanese Patent Laid-Open Publication No. 11-326922.
When manufacturing a liquid crystal display, large-size glass substrates (termed mother glass substrates) have been generally used to simultaneously manufacture a plurality of liquid crystal display devices. In line with the shift to large-size liquid crystal display devices, there has been an increasing tendency for the size of recent mother glass substrates to be enlarged. If the mother glass substrates are enlarged, it is difficult to apply uniform pressure to the entire glass substrates due to insufficient parallelism between upper and lower level blocks of an assembling device. Consequently, adhesion between the sealant and the mother glass substrates is reduced.
For example, in a case where the method disclosed in Japanese Patent Laid-Open Publication No. 11-326922 is applied to the manufacture of liquid crystal display devices which employ large-size mother glass substrates, the sealant is applied as shown in FIGS. 4A and 4B. Hereinafter, patterns which enclose display areas and include a sealant disposed at closest positions of the display areas are referred to as main seal patterns, and patterns which include other sealant are referred to as dummy seal patterns. In FIG. 4A, each of two display areas of the mother glass substrate 95 is individually enclosed by the main seal pattern 96, and these main seal patterns 96 are enclosed by a dummy seal pattern 97, which is formed along the edge of the mother glass substrate 95. In FIG. 4B, each of the two display areas in the mother glass substrate 95 is individually enclosed by the main seal pattern 96, and these main seal patterns 96 are respectively enclosed by dummy seal patterns 98.
However, there is a possibility that the formation of the dummy seal pattern 97 as in FIG. 4A causes faulty display in both liquid crystal display device if adhesion between two mother substrates and the sealant constituting the dummy seal pattern 97 is insufficient. Moreover, the inventors of the present invention has proved in experiments that the formation of the dummy seal patterns 98 as in FIG. 4B is likely to cause insufficient adhesion of the sealant in the central portion (portion enclosed by a broken line in the drawing) of the mother glass substrate 95.